Tumors and lesions are types of pathological anatomies (e.g., tumors, lesions, vascular malformations, nerve disorders, etc.) characterized by abnormal growth of tissue resulting from the uncontrolled, progressive multiplication of cells that serve no physiological function. A non-invasive method for pathological anatomy treatment is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source is changed, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term radiotherapy refers to a procedure in which radiation is applied to a target region or volume of interest (“VOI”) for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (or fraction) (e.g., 100-200 centiGray (cGy)) and short treatment times (e.g., 10 to 30 minutes per fraction) over a period of days (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted by the magnitude of the radiation.
The two principal requirements for an effective radiation treatment system are homogeneity and conformality. Homogeneity is the uniformity of the radiation dose over the volume of the target (e.g., pathological anatomy such as a tumor, lesion, vascular malformation, etc.) characterized by a dose volume histogram (“DVH”). An ideal DVH for the pathological anatomy would usually be considered to be a rectangular function, where the dose is 100 percent of the prescribed dose over the entire volume of the pathological anatomy. An ideal DVH for a critical region (i.e., an important region or structure within the patient to avoid exposing to radiation) would have a rectangular function where the entire volume of the critical anatomical structures receives zero dose. In practice these ideal dose distributions are not achieved, and a range of dose is delivered to both the pathological and critical anatomical structures.
Conformality is the degree to which the radiation dose matches (conforms) to the shape and extent of the target VOI in order to avoid damage to critical adjacent structures. More specifically, conformality is a measure of the amount of prescription (Rx) dose (amount of dose applied) within a target VOI. Conformality may be measured using a conformality index (CI)=total volume at >=Rx dose/target volume at >=Rx dose. Perfect conformality results in a CI=1.
Treatment quality, which may be measured based on homogeneity, conformality, and risk of complications generally improves with the larger number of spatial nodes from which a radiation source can deliver the prescribed radiation dose. Providing a large number of spatial nodes enables the radiation source to have greater flexibility to irradiate the VOI from a larger sample of directions and angles, thereby increasing its ability to avoid critical structures while accurately delivering the prescribed dose to the target VOI. However, since radiation treatment systems typically use large, expensive equipment, the radiation source cycles through the entire set of spatial nodes along known safe interconnecting paths. Even though a particular treatment plan may call for delivery of radiation from only some of the available spatial nodes, the radiation source still visits each and every node along its known safe path to ensure a collision with the patient or other equipment does not occur.
Accordingly, the larger the number of spatial nodes the longer the treatment time. A smaller node set having fewer spatial nodes enables faster treatment time, but often at the expense of less flexibility and therefore potentially lower treatment quality. Accordingly, conventional techniques must balance treatment flexibility and quality versus treatment time.